I. Field of the Invention
This invention relates generally to signal processing apparatus, and more particularly to apparatus for removing noise signal components from noisy analog electrocardiograph (ECG) signals derived from body contacting electrodes without significantly distorting the ECG signal itself
II. Discussion of the Prior Art
Patients having an implanted cardiac rhythm management device, such as a pacemaker or a pacemaker/defibrillator, are periodically monitored using a telemetry link to read out information from the implanted device to an external programmer. Real-time telemetry of measured data is available in most modern pacer systems, including an assessment of battery voltage, current and impedance as well as lead impedance. These are useful measures in diagnosing and anticipating problems with a pacer system. Telemetry of event counters and histograms may be especially useful in evaluating the percentage of time that a patient spends in a particular rhythm or at a particular rate. In this fashion, electrograms having paced events, sensed events and marker channel data can be read out using a telemetry wand positioned over the implanted device.
It is also quite common to affix surface electrodes to the patient to obtain ECG surface data for presentation on a display screen or strip chart recorder for comparison purposes. The analog output signals from the surface electrodes are often contaminated by ambient noise, such as 60 Hz noise from lighting and other AC power sources. Muscle and motion artifacts may also be a source of noise contamination to the ECG signal resulting from cardiac depolarization or repolarization.
It is common practice to utilize filtering techniques in an attempt to isolate the ECG signal itself from whatever noise may be superimposed thereon. However, to date, insufficient attention has been paid to the differences between devices for addressing bradycardia and those for addressing tachycardia. While pacemakers for treating bradycardia deal mostly with conductance problems of the heart, anti-tachy devices deal with the problems in the heart tissue. The surface ECG data for brady devices exhibits a lower heart rate and wider QRS complexes and PR intervals, whereas the ECG data for anti-tachy devices has higher heart rates, shorter QRS complexes and some high frequency oscillations. Thus, a higher quality low-pass filter is required for stripping noise components from ECG signals of anti-tachy devices to address the high frequency components of the ECG signal. It is a principal object of this invention to provide an improved, low-pass filter capable of removing noise signal components from higher frequency ECG signals obtained from tachycardia patients without distorting the ECG signal itself.
A low-pass filter in accordance with the present invention comprises a plurality of digital filter stages, each with a different cut-off frequency, and connected in tandem between a signal source and a signal output point. Each of the filter stages includes a time delay buffer operative to receive digital data representing sampled values of an analog input signal for at least temporarily storing said sampled values for predetermined time intervals. Responsive to the digital data representing sampled values contained in the time delay buffer is a means for developing an average over time noise signal component that falls within a predetermined frequency band. Each of the plural stages also includes a means for selectively subtracting the average over time noise signal component from a sampled value resident in the time delay buffer to thereby effectively remove the noise component from the stored sampled value.
The average over time noise signal component is preferably derived using a high-pass filter adapted to receive the digital data representing the sampled values stored in the time delay buffer and having a dynamic noise level calculator operative to receive filtered data from the high-pass filter and averaging the filtered data over time.